This invention relates generally to level detecting circuits and, more particularly, is directed to a level detecting circuit of the logarithmic compression type.
Noise reduction circuits for reducing noise and distortion which accompany a reproduced information signal are well-known in the art. Such noise reduction circuits are designed to increase the dynamic range of the signal that can be recorded and reproduced from a recording medium such as a magnetic tape. Such noise reduction circuits generally incorporate an encoding process which compresses the level of the information signal prior to recording the signal on the recording medium, and a decoding process which expands the level of the information signal, during the reproducing operation, with a characteristic which is complementary to the compression characteristic. As a result, various restrictions imposed on the dynamic range of the information signal by the signal transmission paths and the recording medium can be eliminated.
One such noise reduction circuit uses a transmission circuit having a variable compression/expansion characteristic which is dependent on the level and/or frequency of the input information signal. Such transmission circuit has a gain controlled amplifier, such as a voltage controlled amplifier, which effects the aforementioned compression and expansion operations, and a level detecting circuit which supplies a control voltage corresponding to the input information signal to the voltage controlled amplifier for controlling the variable compression/expansion characteristic.
In such systems, when the level of the input information signal is abruptly increased, the resulting reproduced output signal has a corresponding overshoot portion which is substantially greater than the desired level of the output signal. The time within which this overshoot portion falls back to its desired level is termed the attack time or rise time constant. However, it becomes difficult to choose a correct attack time since an attack time which is too long will distort the sound which is eventually reproduced and an attack time that is too short will result in a clicking noise in the reproduced sound. In like manner, when the input signal level falls from a high value to a low value, a negative overshoot occurs and the time within which the level of the signal returns from the overshoot level to its desired level is termed the recovery time or fall time constant. Accordingly, a sophisticated "forward masking effect" is used to optimally determine the respective time constants. An optimum attack time is therefore set in the range of approximately 100 .mu.sec. to 10 msec. The recovery time is optimally set for a comparitively long time, for example, in the range from several ten msec. to several hundred msec., that is, at least one hundred times the attack time.
When an input information signal is supplied to the noise reduction circuit, noise which is generated in the magnetic tape and which is noticeable is superimposed upon the output of the system. Since the level of the generated noise is generally much less than that of the input information signal, the noise is masked by the input signal. However, in the case where a signal, such as a tone burst signal, is constantly supplied to the noise reduction circuit, and is then suddenly dropped at a predetermined instant of time, the input information signal supplied to the circuit is sharply attenuated or blocked. On the other hand, the generated noise is not attenuated instantaneously, but rather, is attenuated with a definite time constant determined by the fall time constant of the level detecting circuit. Accordingly, this portion of the noise is not directly masked by the input information signal. Generally, however, when a high level signal is blocked or sharply attenuated at such predetermined instant of time, the human ear will not regain its sensing capacity with respect to a low level signal, such as the aforementioned noise signal, until a predetermined lapse of time. In such case, if the attenuation of noise accompanying the sharp attenuation or blocking of the input information signal is effected during the forward masking period, that is, typically from 100 msec. to 200 msec., the accompanying noise is not sensed by the human ear. This phenomenon is generally referred as a "noise modulation" phenomenon. Accordingly, it is therefore desirable to set the fall time constant of the level detecting circuit to about 100 msec.
On the other hand, if the fall time constant is set at approximately 100 msec., ripple components contained in the detected output increase, resulting in an increase in harmonic distortion. In particular, the detected output of the noise reduction circuit contains ripple components which are comprised mainly of the fundamental wave of the input information signal where half-wave rectification is performed and the second harmonic wave of the input information signal in the case where full-wave rectification is performed. In such case, the level of the ripple components is substantially inversely proportional to the fall time constant and frequency. With the above-described noise reduction circuit, the gain control amplifier which is controlled by the level detecting circuit functions as a multiplier such that a second harmonic wave is generated in response to the fundamental component of the ripple and a third harmonic wave is generated in response to the second harmonic component of the ripple, thereby resulting in harmonic distortion.
Although harmonic distortion does not pose as a problem in a simplified noise reduction circuit which effects the aforementioned compression and expansion operations over only a high frequency region, significant problems are presented with a high performance noise reduction circuit, which in addition, effects a noise reduction operation over a low frequency region. In the latter case, it is therefore necessary to extend the aforementioned fall time constant which, in turn, renders it impossible to set an optimum fall time constant for the aforementioned noise modulation phenomenon.
To overcome this problem, it has been proposed to utilize a gain control circuit which is controlled by a rectangular signal detecting circuit, as described more fully in U.S. patent application Ser. No. 06/246,392, filed Mar. 23, 1981, having a common assignee herewith, and the disclosure of which is incorporated into this application. However, in this latter circuit, the dynamic range of the rectangular signal detecting circuit is determined by the supply voltage and the off-set voltage of the circuit, and thereby has a theoretical upper limit for the dynamic range of about 60 dB. Accordingly, where it is required that the dynamic range used with a noise reduction circuit exceeds 40 to 50 dB, it is more practical to use an exponential-to-logarithmic conversion circuit or a logarithmic compression circuit having a dynamic range greater than 60 dB for the level detecting circuit which produces the control voltage for the gain control amplifier. A level detecting circuit of the logarithmic compression type is described in U.S. patent application Ser. No. 06/325,207, filed Nov. 27, 1981, having the same inventorship and a common assignee herewith, and the disclosure of which is incorporated herein by reference. However, even in the aforementioned level detecting circuit of the logarithmic compression type, no means are provided for controlling the fall time constant to account for both low frequency harmonic band distortion and the aforementioned noise modulation phenomenon.